Thermoplastic polyurethanes belong to the class of the thermoplastic elastomers. Thermoplastic elastomers have a uniform structural principle regardless of their chemical composition. They are block copolymers in which hard blocks are connected to soft blocks in a polymer chain. Hard blocks are understood as being polymer segments whose softening temperature (that is to say glass transition temperature and/or crystallite melting temperature) is far above the use temperature. Soft blocks are understood as being polymer segments having softening temperatures far below the use temperature. The hard blocks form physical linkages between the soft polymer blocks, which linkages are reversibly cleaved during thermoplastic processing and can be re-formed upon cooling. Typical examples of thermoplastic polyurethanes are styrene-butadiene block copolymers having hard polystyrene blocks (glass transition temperature approximately 105° C.) and soft polybutadiene blocks (glass transition temperature approximately −90° C.).
Thermoplastic polyurethane elastomers (TPU) have been known for a long time. They are important technically owing to the combination of high-quality mechanical properties with the known advantages of inexpensive thermoplastic processability. By using different chemical structural components, it is possible to achieve a wide range of variation of mechanical properties. An overview of TPUs, their properties and uses is given, for example, in Kunststoffe [plastics material] 68 (1978), pages 819 to 825 or Kautschuk, Gummi, Kunststoffe [unvulcanized rubber, vulcanized rubber, plastics material] 35 (1982), pages 568 to 584. TPUs usually contain as the semi-crystalline hard phase the reaction product of an organic diisocyanate with a low molecular weight diol and as the amorphous soft phase the reaction product of an organic diisocyanate with a higher molecular weight diol, for example a polyester, polyether or polycarbonate diol having molecular weights of usually from 500 to 5000 g/mol. A wide variety of property combinations can purposively be established via the polyols. In order to accelerate the formation reaction, catalysts can additionally be added. In order to adjust the properties, the structural components can be varied within relatively wide molar ratios. Molar ratios of polyols to diisocyanates to chain extenders of from 1:3:2 to 1:10:9 have been found to be successful in many cases. Products in the range of from 60 Shore A to 75 Shore D can thereby be obtained.
Thermoplastically processable polyurethane elastomers can be synthesized stepwise by the prepolymer metering process. Alternatively, the reaction can take place by the stepwise ester-split process, in which a portion of the polyol is metered in with the chain extender, or by the simultaneous reaction of all the components in one stage, the so-called one-shot metering process, or by production in a reaction extruder.
TPUs can be produced continuously or discontinuously. The most well known commercial production processes are the belt process (GB 1 057 018 A) and the extruder process (DE 19 64 834 A, DE 23 02 564 A, and DE 20 59 570 A).
For the use of TPUs in seals, the following chemical, static and dynamic properties inter alia are relevant: resistance to the media that are used or that occur in the surroundings, temperature resistance at high temperatures, flexibility at low temperatures, pressure resistance, extrusion resistance, wear behavior and relaxation behavior. In polyurethanes, these properties are determined very greatly by the nature of the polyols used. Each class of polyol materials has specific advantages and disadvantages owing to its chemical structure.
Polyols based on polycarbonates provide high mechanical strength and, owing to the high soft segment melting range which is produced by polycarbonates, good properties at high use temperatures. Furthermore, polycarbonates exhibit excellent hydrolytic stability. However, the low-temperature properties are relatively poor on account of the high glass transition temperature of polycarbonates. In addition, polycarbonate-based TPUs have little flexibility owing to the inherent tendency of the material to crystallization.
Polyols based on polycaprolactones, on the other hand, permit a balanced property profile in respect of low-temperature properties, hydrolytic stability and mechanical strength, but they do not achieve peak values in any of the mentioned properties. However, since the ester bond has a tendency to hydrolysis in acidic or alkaline media, polyurethanes based on polycaprolactone polyols are less hydrolytically stable than materials based on polyethers or polycarbonates.
Polyols based on polyesters produced by polycondensation have high stiffness at low temperatures. In addition, they have low hydrolytic stability. However, such polyester polyols exhibit very good strength and also very good wear and abrasion properties.
Polyols based on polyethers lead to very good hydrolytic stability because of the stability of the ether bond. Moreover, the very low glass transition temperature in polyether polyols leads to substantially lower possible use temperatures and substantially better low-temperature properties in comparison with polyester and polycarbonate polyols, as well as higher flexibility. The disadvantage of polyether polyols is, however, that, of all the known polyols, they lead to the lowest mechanical strengths and at the same time have very poor high-temperature properties.
Copolymers of at least two of the mentioned polyol classes, which can be obtained, for example, by a stepwise polymerization of two different monomers that are capable of copolymerization or by polymerizing a different monomer onto an already existing polyol, combine the properties of the polyol classes used and allow better control of the rate of reaction than does a mixture of two different polyols. Furthermore, the use of copolymers leads to better constancy in the properties of the TPU materials that are produced than does a comparable polyol mixture.